1. Field of Invention
The present invention relates generally to rotary shaft angular position and speed sensors.
More specifically, the invention relates to contactless angular sensors adapted to provide linear output signals proportional to shaft speed and position for full 360 degree rotations of the shaft, and which, while suitable for use with other rotary shaft elements, is particularly useful in connection with sensing the angular position and speed of a torque transmitting shaft extending therethrough.
2. Description of Prior Art
Shaft angular position sensing has historically been accomplished using potentiometers, synchros, or resolvers that rely on low reliability electrical contact arrangements such as electrical brushes and wipers. Shaft rotational speed sensing has historically been accomplished utilizing magnetic tachometers which also rely on brush contacts. Newer technologies for angular position and speed sensing include optical encoders which are unreliable in low temperature, moist environments. The need for high reliability shaft angle sensing for aircraft control surfaces and closed loop actuators has led to the application of rotary variable differential transformers (RVDTs). Unfortunately, these sensors are substantially more expensive and require sophisticated and expensive demodulation electronics to obtain useable output signals. Shaft speed sensing for high-reliability applications have often utilized magnetic pickoffs which sense the frequency of passing of a gear tooth or lobe. For reliable implementation, these sensors also require relatively expensive electronics packages.
As a result, recent efforts to achieve a lower-cost, yet reliable and accurate apparatus for sensing angular position and speed of a rotary shaft have included attempts to utilize less expensive sensor elements such as Hall effect devices or magnetoresistive (MR) sensors that are capable of generating an electrical output signal when exposed to a rotating magnetic field. Hall effect sensors utilize a current-carrying semi-conductor membrane to generate a low voltage perpendicular to the direction of current flow when subjected to a magnetic field normal to the surface of the membrane. Magnetoresistive sensors utilize an element whose resistance changes in the presence of a changing external magnetic field.
One group of prior art using these magnetic field sensors provide an output which is digital in nature, generating pulses as a functions of shaft speed or discrete signals for incremental shaft angles. Nichols, U.S. Pat. No. 4,373,486, Schroeder, U.S. Pat. Nos. 5,731,702 and 5,754,042, and Seefeldt, U.S. Pat. No. 5,744,950, use permanent-magnet biased Hall effect devices and magnetoresistive sensors, respectively, to sense the passage of notches on a shaft-driven wheel for engine ignition control and shaft speed control. Kajimoto, U.S. Pat. No. 5,574,364, utilizes magnets imbedded into or polarized into the surface of a rotating wheel to provide a changing magnetic field direction as the surface of the wheel passes the sensors. The digital output signals require use of a microcomputer to practically implement their sensing and control functions. None of the above arrangements provide for an analog output representative of shaft speed.
Some devices use magnetic field sensors to provide analog output signals as a magnet attached to a shaft is rotated. van den Berg, U.S. Pat. No. 5,650,721, shows a two-pole rectangular bar magnet rotating over a giant MR layer. The rotation of the transverse field between the poles creates a unique, sine-wave-shaped analog output over 180 degrees of rotation. However, linear output range is less than 60 degrees. Lochmann, U.S. Pat. No. 6,064,197, adds a Hall effect device to sense axial field direction and provide a unique, but nonlinear, signal over 360-degrees. Andraet, U.S. Pat. No. 5,796,249, proposes the integration of at least three MR Wheatstone bridges under the transverse field of a bar magnet to provide a set of nonlinear outputs that can be used to calculate a unique shaft angle. Häberli, International Publication WO98/54547, proposes a similar scheme utilizing two pairs of Hall effect sensors located on diagonals under a square magnet to generate approximate sine and cosine signals as the shaft and magnet are rotated, and from which the shaft angle is calculated. Muth, U.S. Pat. No. 5,602,471, proposes use of multiple MR bridges to generate a variety of phase-spaced sinusoidal signals. The signals are forced to saturate within their linear range and then added to provide a summed output which is overall a linear function of shaft rotations, but which can exhibit a variety of gain variations and discontinuities. None of these analog sensors lend themselves to being packaged around an axially continuing shaft, a feature desirable for compactly integrating angular sensor function into an electromechanical actuator or other torque carrying device.
Other analog shaft angle sensors using magnetic flux sensors have attempted to increase the linear operating range of typically sinusoidal signals by shaping the magnets or pole pieces. Wu, U.S. Pat. No. 5,159,268, has generated a bell or oblong shaped two-pole magnet to get a linear range approaching 180-degrees. Rountos, U.S. Pat. No. 5,850,142, uses a pair of convex magnets and a spherical pole piece to generate a linear range of up to plus and minus 30 degrees for joysticks. Dawley, U.S. Pat. No. 4,719,419, uses a monopolar annular magnet, either mounted eccentric to the shaft or nonuniformly magnetized, to create a useable linear output of +/−45 degrees. Nakamura, U.S. Pat. No. 4,425,557, and Tomczak, U.S. Pat. No. 4,570,118 incline the sensor magnets relative to the axis of rotation in an attempt to improve output linearity. Luetzow, U.S. Pat. Nos. 5,444,369 and 6,137,288 and Herden, U.S. Pat. Nos. 5,861,745 and 6,130,535 use a combination of shaped magnets, pole pieces, and axis offsets to get a linear output range approaching 180-degrees.
Overall, the prior contactless shaft angular position and speed sensing apparatus are either adapted to provide only a digital output signal that must be further processed or manipulated with additional components, require magnetic elements manufactured with non-standard shapes, do not provide a useful linear operating range, or do not lend themselves to being packaged such that the sensed shaft can extend fully through the sensor components.
Thus, it is apparent that there is a need for a high-reliability, low cost, rotary shaft sensor that is simple to manufacture, can provide linear output of both angular position and speed over a full 360 degrees of rotation, and can be packaged around a torque-carrying element such as associated with a typical rotary actuator.